The use of instrumentation to back analyze rock mass modulus during a cavern construction in Hong Kong

Abstract

A Centennial Campus is being developed at the western side of The University of Hong Kong Campus. The scope of this infrastructure project comprises the design, construction and commissioning of two new salt water and two new fresh water service reservoirs. Cavern was excavated in mostly tuff with intercalated tuffaceous sandstone to accommodate the new salt-water reservoir in a twin-cell tunnel system. With the requirements of minimal damage and disturbance to the rock mass during the excavation, it provided an ample opportunity to study the stress induced by the cavern as the excavation approached a virgin zone. This paper presents an evaluation of the magnitude of stresses acting on the crown of the large span tunnel at different stages of ground movement.

Back analysis using two-dimensional finite element analysis was carried out based on the observed stress change and deformation resulted from approximately 6 m high top heading tunnel excavation to assess the deformation modulus of the bedrock and in-situ stress condition.

The observed monitoring records also demonstrated that approximately 2 mm of vertical deformation was mobilized to provide an efficient temporary support to the tunnel crown. The induced stress calculated was in good agreement with those measured if a constant stress field (Ko > 1) was applied where the in-situ stresses were obtained from a nearby hydro-fracturing test.

This paper presents a case study of the rock mass behavior due to excavation of an underground opening.